Pat lamb shoulder chunks dry and season well with salt and pepper. Heat vegetable oil in a Dutch oven or medium pot over medium-high heat. Once pan is hot add lamb and sear on all sides until golden-brown. Transfer to a plate and set aside.
Add onion, carrots, and parsnips to pan drippings and saute over medium-high heat 3-5 minutes until onion is translucent. Add garlic and cook 1-2 minutes until fragrant.
Pour Guinness into pot and use a wooden spoon to scrape up any browned bits. Bring to a simmer over medium-high heat, then reduce heat to medium-low and allow to simmer 3-5 minutes until Guinness is reduced a bit.
Add lamb, baby potatoes, beef broth, cornstarch-water mixture, thyme sprigs, and bay leaf to pot and stir until evenly combined. Bring to a simmer over medium-high heat, then reduce heat to medium-low, cover pot, and allow to simmer 45-60 minutes, stirring occasionally until lamb and vegetables are fork-tender.
Remove thyme sprigs and bay leaf and discard. Season stew with salt and pepper to taste and serve warm with bread for dipping. Enjoy!